Maria is the family's maid. She is very subservient and always keeps her head bowed and her eyes on the ground. This is shown in the first paragraph in the book and when the young blond soldier looks into Bruno's room (pg 18) Bruno was told by his mother to always treat Maria respectfully and not imitate the way Father spoke to her. She specifically takes care of Bruno, especially in the new house because there are other servants who come with the house and take care of the other duties. Bruno feels he can talk to Maria. He has always liked Maria and felt as if she was one of the family. (pg 17) However, Maria knows her place and tries to help Bruno see that his father has an important job. Even though Maria has been with them since he was three, he does not want her to see him cry. He holds back tears when they are having a conversation in the room at the new house because he doesn't want her to think he is a baby. (pg 19)
You learn more about Maria in Chapter Six. She has a definite history with the family.